Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature.
Various types of photoconductive insulating elements are known for use in electrophotographic imaging processes. In many conventional elements, the active components of the photoconductive insulating composition are contained in a single layer composition. This composition is typically affixed, for example, to a conductive support during the electrophotographic imaging process.
Among the many different kinds of photoconductive compositions which may be employed in typical single active layer photoconductive elements are inorganic photoconductive materials such as vacuum evaporated selenium, particulate zinc oxide dispersed in a polymeric binder, homogeneous organic photoconductive compositions composed of an organic photoconductor solubilized in a polymeric binder, and the like.
Another especially useful photoconductive insulating composition which may be employed in a single active layer photoconductive element are the high-speed "heterogeneous" or aggregate photoconductive compositions described in Light, U.S. Pat. No. 3,615,414 issued Oct. 26, 1971 and Gramza et al, U.S. Pat. No. 3,732,180 issued May 8, 1973. These aggregate-containing photoconductive compositions have a continuous electrically insulating polymer phase containing a finely-divided, particulate, co-crystalline complex of (i) at least one pyrylium-type dye salt and (ii) at least one polymer having an alkylidene diarylene group in a recurring unit.
Recently, an especially useful "multi-active," photoconductive insulating composition has been developed which contains a charge-generation layer in electrical contact with a charge-transport layer, the charge-generation layer comprising a multi-phase aggregate composition as described in U.S. Pat. No. 3,615,414 having a continuous, polymeric phase and dispersed in the continuous phase a co-crystalline complex of (i) a pyrylium-type dye salt, such as 2,4,6-substituted thiapyrylium dye salt, and (ii) a polymer having an alkylidene diarylene group as a repeating unit, and the charge-transport layer comprising an organic photoconductive charge-transport material. When a uniform-polarity electrostatic charge is applied to the surface of this multi-active element and the charge-generation layer thereof is subjected to an image wise exposure to activating radiation, the charge-generation layer generates charge carriers, i.e., electron-hole pairs, and injects them into the charge-transport layer which accepts and transports these charge carriers through the multi-active element to form an electrostatic charge pattern at or near the surface of the multi-active element corresponding to the imagewise exposure. The above-described, multi-active element is described in Berwick et al, copending U.S. Pat. application Ser. No. 534,979, filed Dec. 20, 1974 now abandoned continuation-in-part in U.S. Ser. No. 639,039 filed Dec. 9, 1975.
In addition to the above-described "multi-active" photoconductive composition, another useful type of multi-active photoconductive element has been recently developed. This latter multi-active element is described in Mey, U.S. Serial No. 534,978 filed Dec. 20, 1974 now abandoned continuation-in-part in U.S. Ser. No. 641,058 filed Dec. 15, 1975 and contains a multi-phase aggregate composition as described hereinbabove and in Light, U.S. Pat. No. 3,615,414 in electrical contact with an inorganic photoconductive composition, such as selenium.
Each of the above-described multi-active elements have provided distinct advantages over the prior art. For example, the multi-active element described in Berwick et. al., U.S. Ser. No. 534,979 provides, in its preferred embodiments, substantially higher electrical speeds than can be obtained by conventional, single layer aggregate photoconductive compositions as described in Light, U.S. Pat. No. 3,615,414. And, the multi-active element described in Mey, U.S. Ser. No. 534,978, in certain embodiments thereof which employ selenium in the inorganic photoconductor-containing layer, provides improved blue sensitivity over that which can normally be obtained by a conventional, single layer aggregate photoconductive element as described in Light, U.S. Pat. No. 3,615,414.
There remains, however, the problem that many of the high speed multi-active photoconductive compositions of the type described in Berwick et. al. exhibit less than the desired sensitivity to light in the blue region of the spectrum, i.e., light in the range of from about 400 to 560 nm.